The invention relates to a method for manufacturing a thermopile on an electrically insulating substrate, wherein the starting point is such a substrate having thereon a layer of a first electrically conductive material, to which material a first sensitive lacquer coating is applied which is selectively removed using a mask for placing onto this lacquer coating, whereafter at those positions where the lacquer coating is removed a second electrically conductive material is applied to said first material, wherein the second material is better electrically conducting than the first material and the first and second material, connected as thermocouple, generate a certain thermo-voltage, the remaining part of the lacquer coating is then removed and a freshxe2x80x94secondxe2x80x94lacquer coating is applied over the whole film surface which, using a second mask to be laid onto this lacquer coating, is again also selectively removed in the sense that the lacquer coating remains intact above the second material and above determined positions above the first material which are determined via the mask and which do not make electrical contact with the second material, whereafter the first material not coated by lacquer is finally etched away and the lacquer is then removed.
Such thermopiles are applied for instance in sensors for thermophysical instruments such as flow-meters, with which flow rates of gas or liquid flowsxe2x80x94quantities of gas or liquid which flow through a conduit per unit of timexe2x80x94are measured.
The manufacture of thermopiles of the above stated type is described for instance in the European patent No. 760530.
A film of a polyimide material is for instance used as substrate. A layer of the first electrically conductive material is applied to one side thereof. A film thus results consisting of a substrate having thereon a first conductive material and an optionally present glue layer.
Once the film has been degreased, a photosensitive lacquer coating is applied to the side thereof on which the first material is situated, this in the sense that when the lacquer coating is exposed and sets at the exposed locations, the lacquer on the unexposed locations can be washed awayxe2x80x94a so-called negative lacquerxe2x80x94or precisely the reverse: that the lacquer on the exposed positions can be washed away, a so-called positive lacquer. Exposure usually takes place via a mask with the exposure pattern laid on the first material. The lacquer is then developed in the developer medium prescribed by the manufacturer of the lacquer.
The thus obtained film, which except for the parts which must be coated with a layer of the second material, is coated with lacquer, is now subjected to a galvanizing process: stripes of the second material are grown on the uncovered first material. The height of the stripes to be thus produced is found to be limited by requirements imposed during a second coating with lacquer, later in the manufacturing process, on the volume of air bubbles enclosed particularly during application of the second material: about 12 xcexcm above the first material was found to be the maximum attainable.
After the stripes of the second material have been applied the remaining lacquer is removed. This takes place in an alkaline xe2x80x9cstripperxe2x80x9d and for as short a time as possible, in order to prevent the glue with which the first material is glued to the substrate from dissolving in the stripper. The subsequent application of the second lacquer coating, selective removal thereof via a second mask, etching away of the uncovered first material and, finally, removal of the lacquer also form part of the prior art.
The limitation in the possible height of the stripes, and therewith of the ratio of the quantities of the second material to those of the first, is a great drawback of the above described known method. In the case of a layer of the first material of 10 xcexcm thickness and a 12 xcexcm thick layer of the second material (the maximum thickness), said ratio thus amounts to (1:1.2). An increase in the ratio of the thickness of the second material to that of the first material would result in a better functioning thermocouple.
The invention is based on the insight that it is possible using a per se known technique, i.e. so-called xe2x80x9cblind-hole etchingxe2x80x9d, to make thermopiles with a greatly increased ratio of the thickness of the layer of the second material to that of the first material compared with the known thermopiles. Blind-hole etching is understood to mean etching away material to a certain depth: there remains as it were a bottom with a certain thickness.
The method according to the invention has for this purpose the feature that after the first lacquer coating has been selectively removed, at the positions where this has occurred the first electrically conductive materialxe2x80x94preferably constantanxe2x80x94is etched away to a predetermined depth and only then is the second electrically conductive materialxe2x80x94preferably copperxe2x80x94applied respectively introduced at these positions. The polyimide material of the substrate is preferably xe2x80x9cKaptonxe2x80x9d.
For the materials in question the representatives thereof, i.e. constantan, copper and Kapton, will be used in each case hereinbelow. The invention is however not limited to the use of these materials.
By means of etching a type of channel is arranged in the constantan in which the copper is xe2x80x9cgrownxe2x80x9dxe2x80x94the constantan layer thus becomes thinner at this position and the above stated ratio of the thicknesses thus becomes more favourable: a constantan layer of for instance 10 xcexcm thickness can then be etched away to a thickness of for instance 5 xcexcm, whereafter a layer of copper of 15 xcexcm thickness is applied. This results in a ratio of copper thickness: constantan thickness=3:1.
During etching according to the invention the etching depth is often found difficult to define: a desired depth of for instance 5 xcexcm can easily become 4 xcexcm or 6 xcexcm. By starting from a thicker layer of constantan, which is possible when the etching technique is used,xe2x80x94for instance with a thickness of 25 xcexcm, wherein the etching depth can then be a maximum of 18 xcexcmxe2x80x94the influence of this inaccuracy of the etching depth on the relevant ratio can be reduced. An associated thickness of the film is for instance 100 xcexcm.
It is thus possible in theory, with a thickness of the constantan layer of 25 xcexcm, at an etching depth of 18 xcexcmxe2x80x94and therefore with a remaining constantan thickness of 7 xcexcmxe2x80x94and with a xe2x80x9cbuild-upxe2x80x9d of the copper above the constantan of 10 xcexcm, to achieve a thickness ratio of copper: constantan=28:7=4:1. This is however found to be not wholly realizable in practice since the etched channel does not have a wholly rectangular section perpendicularly of its length direction but is slightly sloping. This pattern is repeated in the grown copper, whereby said ratio will be smaller in practice.
It is recommended to first thoroughly clean the etched constantan surface prior to growing copper thereon: fine dirt particles and also air in the stripes can result in gaps in the copper. In a preferred embodiment the method according to the invention is therefore characterized in that, after etching of the constantan and prior to copper-plating, the etched surface is cleaned by first immersing it in a solution of diluted nitric acid, whereafter it is washed with water and dried.
The copper-plating is for instance realized in an acid copper bath for several tens of minutes. It is found possible to thus realize thermopiles with a regularly distributed copper layer, without rough spots.
The invention comprises a thermopile manufactured using the above described method or embodiment thereofxe2x80x94and the invention also comprises flow-meters equipped with a thermopile according to the invention.